Seat belt device

ABSTRACT

A seat belt device includes a spindle  11  for retracting a seat belt  13 , an electric actuator  55 , a pre-tensioner  14  and a power transmission mechanism  20 , wherein the power transmission mechanism  20  can reversibly switch the connection and disconnection between the electric actuator  55  and the spindle  11  before the pre-tensioner  14  is activated, while when the pre-tensioner  14  is activated, the power transmission mechanism  20  non-reversibly interrupts the connection between the electric actuator  55  and the spindle  11.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to JP 2005-146173, filed May 19, 2005and PCT/JP2006/309960, filed May 18, 2006.

FIELD OF THE INVENTION

The present invention relates to a seat belt device, and moreparticularly to a seat belt device which includes an electric actuatorutilizing a motor and a pyrotechnic pre-tensioner actuator (apre-tensioner).

BACKGROUND OF THE INVENTION

Conventionally, in a seat belt device of this type, when there is apossibility of collision, a seat belt (webbing) is retracted by theelectric actuator before collision, and then the possibility ofcollision disappears, the seat belt is put back to the state before thepossible collision was sensed, and when the collision occurs, the seatbelt is retracted by the pre-tensioner (for example, refer to PatentDocument No. JP-A-2003-191819).

SUMMARY OF THE INVENTION

In this way, in the seat belt device which includes the electricactuator which produces power to rotate a spindle and the pre-tensionerwhich produces another form of power to rotate the spindle, when thepre-tensioner is activated, the spindle for retracting the seat belt isdesirably disconnected from the electric actuator.

Consequently, a main object of the invention is to provide a seat beltdevice including a seat belt retracting member such as a spindle forretracting the seat belt, a primary rotation source such as an electricactuator for rotating the seat belt retracting member when it is broughtinto connection with the seat belt retracting member, and a secondaryrotation source such as a pre-tensioner for rotating the seat beltretracting member at faster speeds than the primary rotational source,wherein when the secondary rotation source such as the pre-tensioner isactivated, the connection of the seat belt retracting member such as thespindle with the first rotation source such as a motor can beinterrupted.

According to the invention, there is provided a seat belt deviceincluding:

-   -   a spindle for retracting a seat belt;    -   an electric actuator for generating power for rotating the        spindle;    -   a pre-tensioner for generating another form of power for        rotating the spindle; and    -   a power transmission mechanism for transmitting the power from        the electric actuator to the spindle, wherein the power        transmission mechanism can reversibly switch the connection and        disconnection between the electric actuator and the spindle        before the pre-tensioner is activated, while when the        pre-tensioner is activated, the power transmission mechanism        non-reversibly interrupts the connection between the electric        actuator and the spindle.

In addition, according to the invention, there is provided a seat beltdevice including:

-   -   a spindle for retracting a seat belt;    -   an electric actuator for generating power for rotating the        spindle;    -   a pre-tensioner for generating another form of power for        rotating the spindle; and    -   a power transmission mechanism for transmitting the power from        the electric actuator to the spindle, wherein when the        pre-tensioner is activated, the power transmission mechanism        interrupts the connection between the electric actuator and the        pre-tensioner by making use of the rotation of the spindle by        the pre-tensioner.

In addition, according to the invention, there is provided a seat beltdevice including:

-   -   a seat belt retracting member for retracting a seat belt;    -   a primary rotation source for rotating the seat belt retracting        member when it is brought into connection with the seat belt        retracting member;    -   a secondary rotation source for rotating the seat belt        retracting member at faster speeds than the primary rotation        source; and    -   a power transmission mechanism, wherein the power transmission        mechanism can reversibly switch the connection and disconnection        between the primary rotation source and the seat belt retracting        member before the secondary rotation source is activated, while        when the secondary rotation source is activated, the power        transmission mechanism non-reversibly interrupts the connection        between the first rotation source and the seat belt retracting        member.

Furthermore, according to the invention, there is provided a seat beltdevice including:

-   -   a spindle for retracting a seat belt;    -   an electric actuator for generating power for rotating the        spindle; and    -   a power transmission mechanism for transmitting the power from        the electric actuator to the spindle, wherein    -   the power transmission mechanism has an actuator-side gear to        which the power from the electric actuator is transmitted, a        spindle-side gear which is provided on the spindle's side and an        elastic piece which is mounted either of the actuator-side gear        and the spindle-side gear and which can be brought into        engagement with the other gear and includes a torque limiter in        which when a torque difference which is larger than a        predetermined value is generated between the actuator-side gear        and the spindle-side gear, the elastic piece cancels the        engagement with the other gear so as to move relative to the        other gear so that the torque difference becomes equal to or        less than the predetermined value.

ADVANTAGE OF THE INVENTION

According to the invention, there is provided the seat belt deviceincluding the seat belt retracting member such as the spindle forretracting the seat belt, the primary rotation source such as theelectric motor which is brought into connection with the seat beltretracting member to rotate the seat belt retracting member and thesecondary rotation source such as the pre-tensioner for rotating theseat belt retracting member at faster speeds than the primary rotationsource, wherein when the secondary rotation source such as thepre-tensioner is activated, the connection between the seat beltretracting member such as the spindle and the primary rotation sourcesuch as the motor can be interrupted.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic vertical sectional view which explains a seat beltdevice of a first embodiment of the invention.

FIG. 2 is a cutaway view taken along the line A-A and viewed in adirection indicated by arrows attached to the line in FIG. 1.

FIG. 3 is a vertical sectional view taken along the line B-B in FIG. 1,which explains the operation of a clutch for transmitting power from amotor.

FIG. 4 is a vertical sectional view taken along the line B-B in FIG. 1,which explains the operation of the clutch for transmitting power fromthe motor.

FIG. 5 is a vertical sectional view taken along the line B-B in FIG. 1,which explains the operation of the clutch for transmitting power fromthe motor.

FIG. 6 is a vertical sectional view taken along the line B-B in FIG. 1,which explains the operation of the clutch for transmitting power fromthe motor.

FIG. 7 is a vertical sectional view taken along the line B-B in FIG. 1,which explains the operation of the clutch for transmitting power fromthe motor.

FIG. 8 is an exploded schematic perspective view of the clutch fortransmitting the power from the motor of the seat belt device of thefirst embodiment of the invention.

FIG. 9 is a view corresponding to the vertical sectional view takenalong the line B-B in FIG. 1 which explains a clutch in an inoperativestate of a seat belt device of a second embodiment of the invention.

FIG. 10 a is a view corresponding to the vertical sectional view takenalong the line B-B in FIG. 1 which explains the clutch of the seat beltdevice of the second embodiment of the invention while a retraction by amotor is in operation.

FIG. 10 a is a view corresponding to the vertical sectional view takenalong the line B-B in FIG. 1 which explains the clutch of the seat beltdevice of the second embodiment of the invention while the retraction bythe motor is in operation.

FIG. 10 c is a view corresponding to the vertical sectional view takenalong the line B-B in FIG. 1 which explains the clutch of the seat beltdevice of the second embodiment of the invention while the retraction bythe motor is in operation.

FIG. 11 a is a view corresponding to the vertical sectional view takenalong the line B-B in FIG. 1 which explains the clutch of the seat beltdevice of the second embodiment of the invention when the retraction bythe motor is cancelled.

FIG. 11 b is a view corresponding to the vertical sectional view takenalong the line B-B in FIG. 1 which explains the clutch of the seat beltdevice of the second embodiment of the invention when the retraction bythe motor is cancelled.

FIG. 11 c is a view corresponding to the vertical sectional view takenalong the line B-B in FIG. 1 which explains the clutch of the seat beltdevice of the second embodiment of the invention when the retraction bythe motor is cancelled.

FIG. 12 a is a view corresponding to the vertical sectional view takenalong the line B-B in FIG. 1 which explains the clutch of the seat beltdevice of the second embodiment of the invention while a pre-tensioneris in operation.

FIG. 12 b is a view corresponding to the vertical sectional view takenalong the line B-B in FIG. 1 which explains the clutch of the seat beltdevice of the second embodiment of the invention while the pre-tensioneris in operation.

FIG. 12 c is a view corresponding to the vertical sectional view takenalong the line B-B in FIG. 1 which explains the clutch of the seat beltdevice of the second embodiment of the invention while the pre-tensioneris in operation.

FIG. 13 are views which explain a gear assembly of a seat belt device ofa third embodiment of the invention.

FIG. 14 is a view which explains a gear having a torque limiter in FIG.13.

FIG. 15 are views which explain a torque limiting operation of the gearhaving the torque limiter.

DETAILED DESCRIPTION

Next, seat belt devices of respective embodiments of the invention willbe described by reference to the drawings.

FIG. 1 is a schematic vertical sectional view which describes a seatbelt device of a first embodiment of the invention, FIG. 2 is a cutawayview taken along the line A-A and viewed in a direction indicated byarrows attached to the line in FIG. 1, FIGS. 3 to 7 are verticalsectional views taken along the line B-B in FIG. 1 which describe theoperations of a clutch for transmitting power from a motor, and FIG. 8is an exploded schematic perspective view of the clutch for transmittingthe power from the motor of the seat belt device of the first embodimentof the invention.

A seat belt device 1 of this embodiment includes a spindle 11 forretracting a seat belt (webbing) 13,

-   -   a pyrotechnic actuator (pre-tensioner) 14 connected to the        spindle 11,    -   a motor 55 as an electric actuator and    -   a clutch 20 as a power transmission mechanism for power from the        motor 55 to the spindle 11.

A torsion bar 12 functioning as an energy absorbing mechanism isprovided within the spindle 11 so as to be connected to the spindle 11.

The pretensioner 14 includes a pinion 15 connected to the spindle 11, atube 17 which accommodates balls 16 and a gas generator 18 provided atone end of the tube 17. When explosives are set alighted, the gasgenerator 18 generates gas, whereby the balls 16 accommodated within thetube 17 are strongly pushed out. The balls 16 which are so pushed outthen move along a groove in the pinion 15 so as to rotate the spindle11.

The motor 55 is connected to a gear assembly 50, whereby the rotation ofthe motor 55 is decelerated by the gear assembly 50. The gear assembly50 and the clutch 20 are accommodated within a container defined by alower cover 21 and an upper cover 29.

The clutch 20 includes a joint 24 connected to the spindle 11,

-   -   a latch plate (a ratchet wheel) 27 integrated with the joint 24        to rotate together with the spindle 11,    -   a final gear 51 of the gear assembly 50 which is gear connected        to a rotational shaft of the motor 55,    -   a pawl 32 mounted on the final gear 51 so as to rotate round a        shaft 38 and adapted to be brought into engagement with a tooth        of the latch plate (ratchet wheel) 27,    -   a return spring 39 which is a biasing member for biasing the        pawl 32 in a direction in which the pawl 32 is disconnected from        the latch plate 27,    -   a guide ring 30 provided inside the final gear 51 so as to be        integral with the final gear 51,    -   a clutch wheel 28 including three leg portions 42 which fit into        three holes 41 in the guide ring 30 so as to rotate relatively        at a required angle,    -   a rotor cam 34 mounted on the clutch wheel 28 so as to rotate        round a shaft 36 while mounted fixedly on the clutch wheel 28 by        means of a fixing pin 37, whereby its rotation is prohibited in        such a state that it is fixed to the clutch wheel 28 by the        fixing pin 37, while its rotation is allowed due to the fixing        pin 37 being broken,    -   a friction spring 25 mounted on the lower cover 21 by an end        portion 26 being caused to fit in a recessed portion 22 in the        lower cover 21 and connected with the clutch wheel 28 by virtue        of friction sliding, and    -   bushes 23, 31.

One end of the pawl 32 moves along a cam surface 35 of the rotor cam 34.

In addition, a rib 28 a is formed on the clutch wheel 28 in apredetermined position which extends in a circumferential direction, andone end portion of this rib 28 a is situated in the vicinity of the pawl32 when in an inoperative state where retraction by the motor 55 is notperformed. Then, when the pawl 32 rotates due to a violently vehiclevibrating with the motor 55 in the inoperative state, the rib 28 a isbrought into abutment with the pawl 32 so as to prevent an abruptrotation of the pawl 32 in a direction in which the pawl 32 is broughtinto engagement with the latch plate 27.

Next, the operation of the seat belt device of the embodiment will bedescribed.

When there is a possibility of collision, the seat belt 13 is retractedby the motor 55 before the possible collision, while when thepossibility of collision disappears, the seat belt 13 is brought back tothe state before the possible collision was sensed. When a collisionoccurs, the seat belt 13 is retracted by the pyrotechnic actuator(pre-tensioner) 14 at faster speeds than the retracting speed by themotor before collision, during which when a predetermined or moretension is applied to the seat belt 13, the tension is limited by thetorsion bar 12.

Next, the operation of the clutch 20 will be described.

Firstly, as shown in FIG. 3, when no retraction is performed by themotor 55, the latch plate 27 and the pawl 32 are out of engagement. Onlythe latch plate 27 integrated with the spindle 11 rotates, and normalretracting/stretching of the seat belt 13 can be performed.

As shown in FIG. 4, when the motor 55 rotates in a retracting direction,the final gear 51, which is gear connected to the rotational shaft ofthe motor 55, rotates in a counterclockwise direction (C direction). Thepawl 32, which is mounted rotatably on the final gear 51, rotates to thelatch plate 27 side along the cam surface 35 of the rotor cam 34 so asto start engagement with the latch plate 27.

As shown in FIG. 5, when the pawl 32 is brought into engagement with thelatch plate 27, the rotation of the final gear 51 is transmitted to thespindle 11 via the latch plate 27, whereby the seat belt 13 is startedto be retracted. As this occurs, the rotor cam 34 rotates together withthe clutch wheel 28 while friction sliding relative to the frictionspring 25.

As shown in FIG. 6, when the motor 55 rotates in releasing direction, inresponse to the rotation, the final gear 51 rotates in the releasingdirection (clockwise direction: D direction). While the pawl 32 rotatestogether with the final gear 51, the clutch wheel 28 and the rotor cam34 mounted on the clutch wheel 28, are maintained by the friction spring25. The pawl 32 departs from the latch plate along the cam surface 35 ofthe rotor cam 34 by virtue of the biasing force of the return spring 39,whereby the pawl 32 is disengaged from the latch plate.

As shown in FIG. 7, when the pre-tensioner 14 is activated, the pawl 32is flicked out outwardly by a tooth surface of the latch plate 27 byvirtue of the fast retracting rotation of the spindle 11 and the latchplate 27 integrated therewith. At the same time, the rotor cam 34 ispushed by the pawl 32 and rotates outwardly about the shaft 36 of theclutch wheel 28. As a result, the pawl 32 and the rotor cam 34 are heldon an outer circumferential portion by virtue of the biasing force ofthe return spring. Thereafter the power of the motor 55 is transmittedin no case to the spindle by the clutch 20.

Next, a seat belt device according to a second embodiment of theinvention will be described in detail by reference to FIGS. 9 to 12( c).Note that like reference numerals will be imparted to like portions tothose of the first embodiment, and the description thereof will beomitted or briefly made.

In this embodiment, in place of the fixing pin 37 of the firstembodiment, a hold spring 60 as an elastic member is provided so as tohold a rotor cam 34 a. Namely, the rotor cam 34 a has an end portion 61which extends to an opposite side of a shaft 36 to a side where a camsurface 35 is provided. In addition, the hold spring 60 is built on aclutch wheel 28 so as to be brought into abutment with the end portion61 on the opposite side of the rotor cam 34 a.

By this configuration, the rotor cam 34 a of this embodiment is mountedon the clutch wheel 28 so as to rotate round the shaft 36, and is fixedto the clutch wheel 28 so as not to rotate in such a state that therotor cam 34 a is biased by the hold spring 60. Furthermore, the rotorcam 34 a is allowed to rotate when the biasing by the hold spring 60 iscancelled.

Next, the operation of a clutch 20 a of this embodiment will bedescribed.

Firstly, when no retraction by a motor 55 is performed, as shown in FIG.9, a pawl 32 is disengaged from a latch plate 27 by virtue of thebiasing force of a return spring 39. Due to this, only the latch plate27 integrated with a spindle 11 rotates, whereby the normalretracting/stretching of the seat belt 13 is enabled.

Next, when the motor 55 rotates in a retracting direction, as shown inFIG. 10( a), a final gear 51 gear connected to a rotational shaft of themotor 55 rotates in a counterclockwise direction (C direction). When thefinal gear 51 rotates, a friction spring 25 idly rotates until acircumferential edge portion of a hole 22 a in a lower cover 21.

When the motor 55 rotates in the retracting direction further, as shownin FIG. 10( b), the clutch wheel 28 is fixed by virtue of the frictionalforce of the friction spring 25, whereby only the final gear 51 rotates.In addition, the pawl 32 rotatably supported on a guide ring 30 rotatesto the latch plate 27 side along the cam surface 35 of the rotor cam 34a against the biasing force of the return spring 39, so as to startengagement with the latch plate 27. Furthermore, when the final gear 51rotates, leg portions 42 a of the clutch wheel 28 are brought intoabutment with circumferential edge portions of holes 41 a of the guidering 30, whereby the final gear 51 and the clutch wheel 28 rotatetogether.

In addition, as shown in FIG. 10( c), when the pawl 32 is brought intoengagement with the latch plate 27, the rotation of the final gear 51 istransmitted to the spindle 11 via the latch plate 27, whereby theretracting of the seat belt 13 is started.

In addition, as shown in FIG. 11( a), when the motor 55 rotates inreleasing direction, in response to the rotation, the final gear 51rotates in the releasing direction (clockwise direction: D direction).Along with this, the webbing is stretched out by virtue of a reactionforce from an occupant which is acting on the webbing, and the clutchwheel 28 and the latch plate 27 rotate together by such an extent thatthe friction spring rotates idly. Thereafter, as shown in FIG. 11( b),the clutch wheel 28 is fixed by virtue of the biasing force of thefriction spring 25, whereby only the final gear 51 and the latch plate27 rotate in the releasing direction. The latch plate 27 maintains ameshing state with the pawl 32 while it is rotating in association withthe rotation of the final gear 51.

Then, as shown in FIG. 11( c), when there is no reaction force comingfrom the occupant and the rotation of the latch plate 27 together withthe spindle 11 in the stretching direction ends, the final gear 51rotates further. Accordingly, the engagement between the pawl 32 and thelatch plate 27 is interrupted and an initial state is thereby restored.

In addition, as shown in FIG. 12( a), when the pre-tensioner 14 isactivated in such a state that the motor rotates in the retractingdirection, the pawl 32 is flicked out outwardly by a tooth surface ofthe latch plate 27 by virtue of the fast retracting rotation of thespindle 11 and the latch plate 27 integrated therewith.

Then, as shown in FIG. 12( b), when the pawl 32 is brought into abutmentwith the rotor cam 34 to thereby push the rotor cam 34 outwards, therotor cam 34 rotates about the shaft 36, whereby the pressure from thehold spring 60 built on the clutch wheel 28 is released (refer to FIG.12( c)). Thereafter, the pawl 32 and the rotor cam 34 are held on anouter circumferential portion of the guide ring 30 by virtue of thebiasing force of the return spring 39. Therefore, the rotation of themotor 55 is transmitted in no case to the spindle 11 by the clutch 20.

Consequently, also in the seat belt device of this embodiment, the powertransmission from the motor 55 is cut off by the clutch 20 at the timewhen the pre-tensioner 14 is activated, and the webbing is retracted bythe pre-tensioner 14 without being subjected to power resistance by themotor 55 and the clutch 20. Therefore, it becomes possible to increasethe retracting performance of the pre-tensioner 14.

Furthermore, while an energy absorbing operation is performed by thetorsion bar 12, the power resistance of the motor 55 and the clutch 20is added in no case to the belt stretching load, thereby making itpossible to increase the restraining performance.

Next, a seat belt device according to a third embodiment of theinvention will be described in detail by reference to FIGS. 13 to 15.This embodiment differs from the first embodiment in the configurationof a gear assembly, and like reference numerals will be imparted to theother like constituent portions to those of the first embodiment,whereby the description thereof will be omitted or made briefly.

As shown in FIG. 13, a gear assembly 50 a of this embodiment includesfirst to fourth gears 71, 72, 73, 74, and a tooth surface of the fourthgear 74 is in mesh engagement with a final gear 51. The first gear 71 iscoupled to a motor shaft of a motor 55, and the second gear 72 has toothsurfaces 72 a, 72 b which are brought into mesh engagement with thefirst gear 71 and the third gear 73, respectively. As shown in FIG. 14,the third gear 73 is a gear assembly with a torque limiter mechanismincluding a large diameter side gear (an actuator-side gear) 80, aplurality of limit springs (elastic pieces) 81 and a tubular smalldiameter side gear (spindle-side gear) 82.

The small diameter side gear 82 has a shape in which a gear portion 82 ameshing with the fourth gear 74, and a spring support portion 82 cincluding slits 82 b to which a plurality of limit springs 81 areassembled are coupled together in an axial direction.

The large diameter side gear 80 has a tooth portion 80 a meshing withthe second gear 72 on an outer circumferential surface thereof. Further,the large diameter side gear 80 accommodates the spring support portion82 c and the limit springs 81 of the small diameter side gear 82 in aninterior wall 80 b and a bottom portion 80 c thereof to which grease isapplied.

A plurality of concave locking surfaces 80 d are formed at predeterminedintervals on the interior wall 80 b of the large diameter side gear 80.Projecting portions 81 a formed on the limit springs 81 are brought intoengagement with the locking surfaces 80 d. In addition, the lockingsurfaces 80 d are formed an integer number of times the number of theprojecting portions 81 a.

Next, the operation of the torque limiter mechanism will be described.

In the normal state, as shown in FIG. 13( a), the phases of the largediameter side gear 80 and the small diameter side gear 82 of the thirdgear 73 are held relative to each other. The large diameter side gear 80and the small diameter side gear 82 rotate in the same direction in theretracting direction shown by a solid line or the releasing directionshown by a broken line. Here, when retracting is performed by drivingthe motor 55, in the event that a torque difference larger than apredetermined value is generated between the large diameter side gear 80and the small diameter side gear 82 due to a light collision or brakingwhich does not activate the pre-tensioner 14, as shown in FIG. 15, theprojecting portions 81 a of the limit springs 81 are released from theengagement with the locking surfaces 80 d and then start to slide alongthe interior wall 80 b while being deformed.

Then, due to the projecting portions 81 a being brought into engagementwith the adjacent locking surfaces 80 d, a rotating deviation isgenerated between the large diameter side gear 80 and the small diameterside gear 82, and as shown in FIG. 13( b), the fourth gear 74 and thefinal gear 51 rotate in a belt stretching direction. As a result, thetransmission of excessive torque by the motor 55 is suppressed, wherebythe failure of gear teeth can be prevented and the effect on therestraining performance during energy absorption operation can bedecreased.

In addition, the torque limiter mechanism is built in the third gear 73of the gear assembly 50 a, whereby although it is configured small, thetorque limiter mechanism can increase the limiter torque of the spindle11.

In addition, while the gear assembly 50 a is preferably applied to theseat belt device which includes the power transmission mechanism of thefirst or second embodiment, the application thereof is not limitedthereto. The gear assembly 50 a may be applied to known seat beltdevices.

Additionally, while the limit spring 81 of the embodiment is mounted onthe small diameter side gear 82, the limit spring 81 may be mounted onthe large diameter side 80, so as to be brought into engagement with ordisengagement from the locking surfaces provided on the small side gear82.

Note that the invention is not limited to the embodiments that have beendescribed above but can be modified or improved as required.

In addition, this patent application is based on the Japanese PatentApplication (No. 2005-146173) filed on May 19, 2005, and all thecontents thereof are incorporated herein by reference.

While the above description constitutes the preferred embodiment of thepresent invention, it will be appreciated that the invention issusceptible to modification, variation and change without departing fromthe proper scope and fair meaning of the accompanying claims.

1. A seat belt device comprising: a spindle for retracting a seat belt;an electric actuator which generates a first power for rotating thespindle; a pre-tensioner which can be activated to generate a secondpower for rotating the spindle; and a power transmission mechanism whichtransmits the first power from the electric actuator to the spindle,wherein the power transmission mechanism is capable of reversiblyswitching connection and disconnection between the electric actuator andthe spindle before the pre-tensioner is activated, while when thepre-tensioner is activated, the power transmission mechanism interruptsthe connection between the electric actuator and the spindle.
 2. Theseat belt device as set forth in claim 1, wherein when the pre-tensioneris activated, the electric actuator and the spindle are disconnectedfrom each other by making use of the rotation of the spindle by thepre-tensioner.
 3. The seat belt device as set forth in claim 1, whereinthe electric actuator is connected with the spindle by the powertransmission mechanism, and wherein when the pre-tensioner is activatedwhile the spindle is being rotated by the electric actuator, theelectric actuator and the spindle are disconnected from each other. 4.The seat belt device as set forth in claim 1, wherein the powertransmission system comprises: a primary engagement member which rotatestogether with the spindle; a secondary engagement member which moves inresponse to the rotation of the electric actuator; and a restrictingmember which restricts a movement of the secondary engagement member sothat the secondary engagement member moves towards the primaryengagement member to bring the secondary engagement member intoengagement with the primary engagement member, when the secondaryengagement member moves in response to the rotation of the electricactuator, and wherein when the pre-tensioner is activated, the secondaryengagement member is caused to move in a direction in which thesecondary engagement member moves away from the primary engagementmember by making use of the rotation of the primary engagement member bythe pre-tensioner, and the restricting member is caused to move by themovement of the secondary engagement member to a position where therestricting member is not capable of restricting the movement of thesecondary engagement member.
 5. The seat belt device as set forth inclaim 1, wherein the power transmission mechanism comprises: a ratchetwheel which rotates together with the spindle; a rotary member whichconnects to the electric actuator; a pawl which is mounted rotatably onthe rotary member and is capable of being brought into engagement withthe ratchet wheel; a friction wheel provided so as to be in contact withthe rotary member; and a cam member mounted on the friction wheel by afixing member and along whose cam surface one end of the pawl is made tomove, wherein when the pre-tensioner is activated, the pawl is caused tomove in a direction in which the pawl moves away from the ratchet wheelby the rotation of the ratchet wheel by the pre-tensioner, so that thefixing member of the cam member is broken by the movement of the pawl,so as to prevent the engagement of the pawl with the ratchet wheel. 6.The seat belt device as set forth in claim 5, wherein the cam member isrotatably mounted on the friction wheel, and the rotation of the cammember is prohibited in such a state that the cam member is mounted onthe friction wheel by the fixing member, while the cam member is allowedto rotate due to the fixing member being caused to fail.
 7. The seatbelt device as set forth in claim 5, further comprising a biasing memberwhich biasing the pawl in a direction in which the pawl is released fromthe ratchet wheel, wherein after the fixing member of the cam memberfails, the cam member is biased in a direction in which the cam membermoves away from the ratchet wheel via the pawl by virtue of biasingforce of the biasing member.
 8. The seat belt device as set forth inclaim 1, wherein the power transmission mechanism comprises: a ratchetwheel which rotates together with the spindle; a rotary member whichconnects to the electric actuator; a pawl which is mounted rotatably onthe rotary member and which is capable of being brought into engagementwith the ratchet wheel; a friction wheel provided so as to be in contactwith the rotary member; and a cam member mounted on the friction wheelin such a state that the cam member is biased by means of an elasticmember and along whose cam surface one end of the pawl is made to move,wherein when the pre-tensioner is activated, the pawl is caused to movein a direction in which the pawl moves away from the ratchet wheel bythe rotation of the ratchet wheel by the pre-tensioner, so that thebiasing of the cam member by the elastic member is released by themovement of the pawl, so as to prevent the engagement of the pawl withthe ratchet wheel.
 9. The seat belt device as set forth in claim 8,wherein the cam member is rotatably mounted on the friction wheel, andthe rotation of the cam member is prohibited in such a state that thecam member is mounted on the friction wheel by being biased by theelastic member, while the cam member is allowed to rotate due to therelease of the biasing of the elastic member.
 10. The seat belt deviceas set forth in claim 8, further comprising a biasing member whichbiases the pawl in a direction in which the pawl is released from theratchet wheel, wherein after the biasing of the cam member by theelastic member is released, the cam member is biased in a direction inwhich the cam member moves away from the ratchet wheel via the pawl byvirtue of biasing force of the biasing member.
 11. The seat belt deviceas set forth in claim 5, wherein the friction wheel has a rib adapted tobe brought into abutment with the pawl so as to prevent the engagementof the pawl with the ratchet wheel when the electric actuator remainsinoperative.
 12. A seat belt device comprising: a spindle for retractinga seat belt; an electric actuator which generates first power forrotating the spindle; a pre-tensioner which generates a second power forrotating the spindle; and a power transmission mechanism which transmitsthe power from the first electric actuator to the spindle, wherein whenthe pre-tensioner is activated, the power transmission mechanisminterrupts the connection between the electric actuator and thepre-tensioner by making use of the rotation of the spindle by thepre-tensioner.
 13. A seat belt device comprising: a seat belt retractingmember for retracting a seat belt; a primary rotation source whichrotates the seat belt retracting member when it is brought intoconnection with the seat belt retracting member; a secondary rotationsource which rotates the seat belt retracting member at faster speedsthan the primary rotation source; and a power transmission mechanism,wherein the power transmission mechanism is capable of reversiblyswitching connection and disconnection between the primary rotationsource and the seat belt retracting member before the secondary rotationsource is activated, while when the secondary rotation source isactivated, the power transmission mechanism non-reversibly interruptsthe connection between the first rotation source and the seat beltretracting member.
 14. A seat belt device comprising: a spindle forretracting a seat belt; an electric actuator which generates power forrotating the spindle; a power transmission mechanism which transmits thepower from the electric actuator to the spindle, wherein the powertransmission mechanism comprises an actuator-side gear to which thepower from the electric actuator is transmitted; and a spindle-side gearwhich is provided on the spindle's side and an elastic piece which ismounted either of the actuator-side gear and the spindle-side gear andwhich is capable of being brought into engagement with the other of theactuator side gear and the spindle side gear and includes a torquelimiter in which when a torque difference which is larger than apredetermined value is generated between the actuator-side gear and thespindle-side gear, the elastic piece cancels the engagement with theother gear so as to move relative to the other gear so that the torquedifference becomes equal to or less than a predetermined value.